UNIVERSITY   OF   CALIFORNIA 

COLLEGE   OF   AGRICULTURE 

AGRICULTURAL   EXPERIMENT   STATION 

BERKELEY,  CALIFORNIA 


THE  RELATION  OF  MATURITY  OF 

CALIFORNIA  PLUMS  TO  SHIPPING 

AND  DESSERT  QUALITY 

F.  W.  ALLEN,  J.  R.  MAGNESS  and  M.  H.  HALLER 


Results  of  investigations  conducted  under  cooperative  agreement 
between  the  California  Agricultural  Experiment  Station  and  the 
Bureau  of  Plant  Industry,  United  States  Department  of  Agriculture 


BULLETIN  428 

June,  1927 


UNIVERSITY  OF  CALIFORNIA  PRINTING  OFFICE 

BERKELEY,  CALIFORNIA 

1927 


THE  RELATION  OF  MATURITY  OF  CALIFORNIA 
PLUMS  TO   SHIPPING  AND  DESSERT  QUALITY 

F.  W.  ALLEN,i  J.  E.  MAGNESS2  and  M.  H.  HALLER3 


INTRODUCTION 

Since  California  plums  are  produced  two  to  three  thousand  miles 
from  the  principal  eastern  markets,  the  harvesting  and  shipping  of 
them  is  a  most  important  problem.  Eight  to  twelve  days  must  usually 
elapse  between  the  time  of  loading  the  fruit  into  refrigerator  cars  and 
its  arrival  on  the  market.  From  one  to  three  days  additional  may  be 
necessary  before  it  reaches  the  retail  purchaser.  Under  these  condi- 
tions it  is  necessary,  therefore,  that  highly  perishable  fruits,  such  as 
plums,  be  harvested  and  shipped  before  becoming  tree  ripe,  if  they 
are  to  carry  satisfactorily.  Firmness  is  not  only  essential  for  success- 
ful long-distance  shipment  but  after  the  necessary  rehandling  the  fruit 
must  be  delivered  to  the  retailer  in  a  firm,  sound  condition. 

On  the  other  hand,  if  picked  too  early,  plums  will  not  ripen  with 
good  dessert  quality.  In  general,  the  nearer  tree  ripe  the  fruit  when 
picked,  the  better  the  dessert  quality  when  fully  ripe.  The  commercial 
grower  and  shipper  must  therefore  pick  and  handle  the  fruit  so  it  will 
arrive  in  a  satisfactory  market  condition,  and  at  the  same  time  secure 
the  highest  possible  dessert  quality.  In  view  of  the  insistent  demand 
from  dealers  that  nothing  but  hard  fruit  be  shipped,  there  has 
developed  a  tendency  to  harvest  a  portion  of  the  crop  at  least,  so  early 
as  to  sacrifice  considerable  of  its  eating  quality.  This  investigation 
accordingly  was  undertaken  to  determine  as  accurately  as  possible 
under  existing  transportation  conditions  when  the  leading  plum 
varieties  should  be  harvested. 


1  Assistant  Pomologist,  California  Agricultural  Experiment  Station. 

2  Physiologist,   Bureau    of   Plant    Industry,    United    States    Department    of 
Agriculture. 

3  Junior  Pomologist,  Bureau  of  Plant  Industry,  United  States  Department 
of  Agriculture. 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


OUTLINE    OF    THE    INVESTIGATIONS 

In  commercial  practice,  the  time  of  picking  plums  for  eastern  ship- 
ment has  been  based  almost  entirely  upon  the  color  of  the  fruit.  In 
attacking  this  problem  it  was  planned  to  determine  as  accurately  as 
possible  the  physical  and  chemical  changes  occurring  in  plums  as  they 
ripen  on  the  tree,  and  after  removal  from  the  tree;  and  how  these 
changes  are  associated  with  color  and  desert  quality. 

Changes  studied  as  the  fruit  of  different  varieties  matured  on  the 
tree  included  (1)  increase  in  size,  (2)  change  in  color,  (3)  change  in 
firmness  of  flesh,  (4)  change  in  soluble  solids  and  sugar  content,  and 
(5)  change  in  acidity.  Following  picking,  similar  studies  were  made 
on  fruit  while  being  held  under  conditions  approximating  those  that 
prevail  in  refrigerator  cars  while  in  transit  to  eastern  markets.  The 
shipping  quality,  or  the  way  the  fruit  holds  up  following  picking  at 
various  maturity  stages,  was  also  determined  for  each  variety. 

Fruit  from  Placer  County,  which  county  ranks  first  in  the  state 
in  the  quantity  of  shipping  plums  produced,  was  chosen  for  the  major 
part  of  the  investigations,  Some  fruit  was  also  obtained  from  the  river 
section  of  Sacramento  County.  With  the  assistance  of  a  committee 
of  shippers,  ten  important  varieties,  including  those  that  had  proved 
most  difficult  to  get  to  market  in  satisfactory  condition,  were  selected. 
These  varieties  included  Beauty,  Formosa,  Climax,  Santa  Kosa, 
Burbank,  Wickson,  Duarte,  Diamond,  Giant  and  President.4 

During  the  seasons  of  1924  and  1925  fruit  of  each  variety  was 
secured  from  three  or  four  different  orchards.  While  differences  in 
soil,  amount  and  frequency  of  irrigations  and  other  cultural  factors 
have  some  influence  on  keeping  quality,  any  differences  noted  were 
considered  of  slight  consequence  in  trying  to  determine  picking 
maturity  in  general.  Hence  in  1926  all  varieties  were  secured  from 
a  single  orchard.  This  orchard  was  typical  of  the  Newcastle-Auburn 
section,  all  trees  being  of  full  bearing  age,  growing  in  a  sandy  loam 
soil  and  receiving  frequent  irrigations  during  the  growing  season. 


COLLECTON  AND   HANDLING   OF  SAMPLES 

Fruit  of  each  variety  was  picked  according  to  color.  The  first 
pickings  usually  corresponded  to  the  stage  at  which  the  earliest  com- 
mercial harvest  occurs.     A  second  picking  corresponded  to  recom- 


4  The  three  last-named  varieties  are  of  the  European  group  (Prunus  domestica) 
while  the  other  varieties  belong  to  the  Japanese  group  (Prunus  salicina)  the 
type  of  plums  now  most  largely  grown  in  California  for  fresh  shipment. 


BUL.  428]  MATURITY   OF   SHIPPING   PLUMS  5 

mended  commercial  harvest  color,  while  a  third  picking  was  made  in 
a  more  advanced  or  mature  color  stage  than  that  commercially  recom- 
mended. The  earliest  fruit  from  the  tree  to  reach  a  certain  color 
stage,  and  the  latest  fruit  from  the  tree  to  reach  that  same  stage  were 
also  taken  for  comparison  as  to  their  holding  and  other  qualities. 
From  1%  to  2  crates  of  four  baskets  each  constituted  a  picking.  The 
specimens  were  taken  from  all  parts  of  the  tree. 

Each  sample  picked  was  immediately  packed  in  regular  plum 
baskets  according  to  commercial  practice,  and  shipped  by  express  to 
the  Division  of  Pomology  at  Berkeley  where  the  fruit  arrived  late 
the  same  day  as  harvested. 

The  following  morning,  the  fruit  was  tested  for  firmness  and 
samples  taken  for  analyses.  The  remainder  of  each  picking  was  then 
placed  in  separate  cold  storage  rooms,  held  under  temperatures  com- 
parable to  those  usually  existing  in  the  bottom  half  and  in  the  top  of 
the  load  in  refrigerator  cars,  43°  and  52°  Fahrenheit,  respectively. 
After  12  days,  the  usual  time  required  for  California  fruit  to  reach 
the  New  York  market,  the  fruit  was  removed,  again  tested  for  firm- 
ness and  samples  taken  for  analyses.  The  remaining  part  of  each  pick- 
ing held  under  the  above  temperatures  was  then  placed  under  a 
65-70°  temperature  until  it  became  overripe. 

MATURITY    CHANGES 

CHANGES  IN  PLUMS  AS  THEY  RIPEN  ON  THE  TREE 
Increase  in  Size. — Increase  in  size  of  fruit  during  the  ripening 
period  was  determined  at  approximately  six-day  intervals  from  the 
time  the  fruit  lost  its  dark-green  color  until  it  was  full  color  or  ready 
to  drop  from  the  tree.  Thirty  to  forty  representative  specimens  from 
different  parts  of  the  tree  were  marked  and  the  horizontal  circum- 
ference or  diameters  of  these  measured  at  the  time  of  each  experi- 
mental picking.  From  these  measurements  the  approximate  increase 
in  volume  was  calculated.5 

In  this  experiment  no  effort  was  made  to  secure  the  rate  of  growth 
during  the  early  stages  of  development  but  only  that  from  the  time  the 
earliest  pickings  were  made  until  the  fruit  Was  considered  too  soft  for 
long  distance  shipment.  These  measurements  were  thought  sufficient 
to  indicate  how  long  growth  continued  and  to  what  extent  tonnage 
might  be  lost  by  extremely  early  picking.  The  results  of  some  of 
these  measurements  have  been  averaged  and  summarized  in  table  1. 


s  Assuming  a  spherical  shape  for  the  fruit  and  little  change  in  form  during 
the  period  of  measurement,  the  volume  varies  approximately  as  the  cube  of  the 
diameter. 


UNIVERSITY   OF   CALIFORNIA — EXPERIMENT   STATION 


TABLE   1 

Increase  in  Size  of  Plums 


Variety 


Color  stage 


Average 
diameter 


Number 

of 

fruits 


Between 


Percentage  increase 


Diameter    Volume 


Beauty. 


Formosa 


Santa  Rosa.. 


Climax.. 


Wickson. 


Burbank. 


Duarte.. 


Diamond 


Giant.. 


(a)  Green 

(b)  Straw  tip  to  light  straw... 

(c)  Light  pink  to  pink  tip 

(d)  M  to  Y2  red 

(a)  Straw  tip  to  light  straw.., 

(b)  Full  straw  to  yellow 

(c)  Slight  to  Yz  pink 

(d)  Yl  to  M  Pink 

(a)  Green 

(b)  Straw  tip  to  straw 

(c)  Pink  tip  to  \i  pink 

(d)  %  to  full  red 

(e)  Full  dark  red 

(a)  Green 

(b)  Straw  tip  to  light  straw.. 

(c)  Yl  to  full  straw 

(d)  Pink  tip  to  slight  pink... 

(e)  M  to  full  red 

(a)  Green 

(b)  Straw  tip 

(c)  Light  straw 

(d)  Full  straw  to  yellow 

(a)  Green  to  light  straw 

(b)  Full  straw  to  trace  pink.. 

(c)  Slight  pink  to  34  red 

(d)  Full  yellow  to  %  red 

(a)  Green  to  straw  tip 

(b)  Pink  tip  to  Yz  light  pink 

(c)  Y2  pink  to  pink 

(d)  Full  red 

(a)  Trace  blue  to  blue  tip 

(b)  Yz  to  Vi  blue 

(c)  Full  blue 

(d)  Full  dark  blue 

(a)  Straw  to  trace  pink 

(b)  Slight  to  Y2  pink 

(c)  %  to  full  pink 

(d)  Vi  to  full  red 

(a).  Slight  to  Yz  purple 

(b)  lAto3/i  purple 

(c)  Full  purple 


{centimeters) 

4.04 

4.14 

4.42 

4.67 

4.70 
4.92 
5.38 
5.40 

4.27 
4.38 
4.67 
5.05 
5.23 

4.24 
4.40 
4.63 
4.87 
4.94 

4.04 
4.62 
4.92 
5.09 

4.20 
4.36 
4.53 
4.66 

4.30 
4.60 
4.71 
4.73 

4.02 
4.27 
4.31 
4.46 

4.40 
4.52 
4.68 

4.72 

4.55 
4.58 

4.68 


a-b 
b-c 
c-d 


a-b 

b-c 
c-d 


a-b 
b-c 
c-d 
d-e 


a-b 
b-c 
c-d 
d-e 


a-b 
b-c 
c-d 


a-b 
b-c 
c-d 


a-b 
b-c 
c-d 


a-b 
b-c 
c-d 


a-b 
b-c 
c-d 


a-b 
b-c 


2.4 
6.3 
5.3 


4.5 
8.5 
3.7 


2.5 
6.2 
7.5 
3.4 


3.6 
4.9 
4.9 
1.4 


12.5 
6.1 
3.3 


3.7 

3.7 
2.8 


6.5 
2.3 
0.4 


3.5 
2.3 
3.1 


2.6 
3.5 
0.8 


0.6 
2.1 


7.4 
20,5 
17.1 


14.1 

27.8 
11.5 


7.7 


24.2 
10.5 


11.2 
15.8 
15.4 
4.3 


42.4 
19  4 
10.2 


11.5 
11.8 


20.8 
7.2 
1.3 


10.8 
7.0 
9.6 


7.7 
10.9 
3.0 


2.0 
6.0 


BUL.  428]  MATURITY   OF   SHIPPING   PLUMS  7 

Naturally  the  attention  given  the  trees  in  different  orchards,  par- 
ticularly the  frequency  of  irrigations  or  water  content  of  the  soil  and 
the  size  of  the  crop  on  the  trees  are  factors  influencing  the  rate  of 
growth.  .Under  rather  unfavorable  conditions  instances  were  found 
where  the  fruit  made  practically  no  increase  after  the  first  measure- 
ments were  made.  On  the  other  hand,  the  greatest  gain  over  a  period 
of  ten  to  fifteen  days  was  4.5  per  cent  in  volume  per  day.  The 
average  gain  of  all  the  varieties  studied,  from  all  orchards,  was 
between  1.5  and  2.0  per  cent  by  volume  per  day. 

Increase  in  size  is  usually  slightly  greater  in  the  earlier  stages  of 
ripening — from  the  time  the  fruit  changes  from  a  decided  green  to 
slight  color — than  it  is  in  the  later  color  stages.  Apparently,  however, 
growth  in  plums  does  not  stop  before  they  attain  their  full  color,  which 
is  at  a  later  date  than  the  fruit  of  most  varieties  is  ever  picked  for 
eastern  shipment. 

The  varieties  listed  in  the  above  table  are  given  in  their  approxi- 
mate order  of  ripening  and  it  is  significant  to  note  that  the  later 
varieties  show  a  smaller  daily  gain  in  volume  than  the  earlier  sorts. 
The  probable  explanation  of  this  is  that  they  have  had  a  longer  period 
in  which  to  make  their  growth.  The  significant  feature,  however,  of 
these  differences  is  that  the  earlier  varieties — those  which  are  most 
often  picked  prematurely — are  the  ones  making  the  most  rapid  gains 
when  harvested.  With  these  varieties  five  or  six  days '  delay  in  harvest- 
ing would,  in  many  instances,  not  only  give  fruit  of  a  more  desirable 
size  but  might  easily  result  in  a  10  per  cent  increase  in  the  number  of 
crates  produced. 

While  each  days '  delay  in  picking  results  in  larger  sized  fruit,  and 
a  heavier  tonnage,  the  picking  date  obviously  cannot  be  postponed 
beyond  the  time  when  the  fruit  has  satisfactory  carrying  quality.  In 
the  majority  of  cases  it  is  doubted  if  growers,  under  present  methods 
of  refrigeration,  would  be  justified  in  delaying  picking  later  than  is 
at  present  customary  practice.  However,  at  the  beginning  of  each 
shipping  season  there  are  always  some  who  in  an  effort  to  secure  high 
prices  for  the  first  fruit  on  the  market,  sacrifice  enough  in  size  to 
offset  the  difference  which  a  few  days  would  make  in  price.  As 
discussed  later,  even  more  is  sacrificed  in  quality. 

Change  in  Color. — Within  a  period  of  ten  days  to  several  weeks 
before  becoming  full  ripe,  most  varieties  of  plums  undergo  marked 
changes  in  color.  These  changes,  while  continuous,  are  as  a  matter  of 
practical  convenience  divided  into  several  more  or  less  distinct  stages. 
In  the  Japanese  varieties  the  earliest  of  these  changes  often  spoken  of 
as  "breaking,"  is  when  the  decided  green  of  the  skin  changes  to  a 


8  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

light  whitish  or  yellowish  green.  With  most  varieties  this  yellowish 
green  changes  to  a  more  decided  yellow  or  straw  color,  following 
which  the  plum  gradually  assumes  its  characteristic  yellow  or  red 
color.  The  fruits  are  thus  spoken  of  commercially  as  "straw  tip," 
"slight  color,"  "red  tip,"  "three-fourths  red,"  etc.  With  varieties 
such  as  the  Duarte  there  is  little  or  none  of  the  straw  or  yellow  stage, 
the  characteristic  red  mottled  effect  developing  while  the  skin  is  still 
green.  The  flesh  also  early  assumes  a  deep  red  color.  In  the  blue  or 
purple  varieties,  the  color  changes  are  usually  from  green  to  greenish 
blue  or  reddish  purple  followed  by  dark  blue  or  purple. 

As  color  changes  are  especially  noticeable,  they  have  been  and  are 
used  as  the  most  important  single  test  for  maturity.  The  color  of  the 
fruit  has,  therefore,  been  used  as  the  picking  standard  in  this  investi- 


Fig.  1. — Use  of  pressure  tester  in  determining  firmness  of  plums. 

gation,  and  the  firmness,  chemical  changes,  and  shipping  quality  has 
been  determined  at  certain  definite  color  stages.  Detailed  data  of 
maturity  changes  for  all  varieties  studied  are  given  in  table  2. 

Softening  of  the  Flesh. — With  the  increase  in  color  of  fruit  there 
is  normally  softening  of  the  flesh.  With  such  fruits  as  plums,  soften- 
ing is  an  outstanding  characteristic  of  the  ripening  process.  As  soft 
fruit  is  unsuitable  for  long  distance  shipment,  its  firmness  when  picked 
is  intimately  associated  with  carrying  quality. 

Firmness  of  the  fruit  at  time  of  picking  was  determined  by  means 
of  the  pressure  tester  illustrated  in  figure  1,  and  described  in  detail 
in  a  previous  publication.6  The  pressure  in  pounds  required  to  force 
a  plunger  point  %e  inch  in  diameter  into  the  flesh  of  the  fruit  to  a 
depth  of  5/1Q  of  an  inch,  was  measured.  Fifteen  to  twenty  plums  were 
used  for  each  determination.  Each  fruit  was  tested  twice,  once  on 
each  cheek,  the  skin  having  first  been  removed. 


6  Magness,  J.  R,  and  George  F.  Taylor.     An  improved  type  of  pressure  tester 
for  the  determination  of  fruit  maturity.     U.  S.  D.  A.  Cir.  350:1-8.     1925. 


Bul.  428] 


MATURITY    OF    SHIPPING    PLUMS 


The  average  results  of  these  tests  on  fruit  from  each  district  studied 
are  shown  in  column  4  of  table  2.  The  averages  of  all  the  tests  taken 
at  the  same  color  stage  are  also  given. 

As  would  be  expected  there  is  considerable  variation  in  the  firmness 
of  different  varieties  when  in  a  similar  color  stage.  In  all  varieties 
tested,  however,  as  shown  by  the  data  in  table  2,  there  is  a  marked 
softening  of  the  fruit  as  color  increases  and  maturity  advances.  The 
variety,  Beauty,  for  example,  illustrated  in  the  graph,  figure  2,  showed 


I 

Beauty 

•5 

*»    IK 

1 

\ 

\     _5j 

nluble   Solid 

s^**^ 

\ 

V  to 

<? 

1. 

<ow 

!• 

_      Ac/cJ 

o 

Green  -to 
Strata  Tip 


Si-raw  -fo 
Pink  Tip 


Straw  +o 
Red  Tip 


Color    Stages 


J     1 


Green  +o         Wnjw  -to  Greenish  Yellow 

Straw  Tip     Greenish  Yellow        to  Bed  Tip 


r 

Bed 


Color  Stages 


Fig.  2. — Showing  physical  and  chemical  changes  in  Beauty  and  Climax 
plums  as  they  ripen  on  the  tree. 

an  average  pressure  test  above  13  pounds  in  the  straw  tip  stage,  while 
in  the  red  tip  stage  the  average  was  only  slightly  over  6  pounds. 
Burbank  tested  above  20  pounds  in  the  straw  tip  to  light  straw  stage, 
and  under  11  pounds  when  yellow  to  one-fourth  red.  Other  varieties 
show  similar  rapid  softening  as  maturity  advances. 


10  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

In  a  number  of  lots,  fruit  in  a  similar  color  stage,  from  the  same 
trees,  was  picked  several  days  apart.  The  earliest  plums  to  reach  a  cer- 
tain color  stage  are  usually  those  on  the  outside  of  the  tree,  where 
they  are  well  exposed  to  the  light.  The  latest  fruit  on  the  tree  to 
reach  any  fixed  color  stage  is  usually  that  from  the  lower  branches 
and  shaded  portions.  When  fruit  of  later  pickings  but  of  a  similar 
color  stage  was  compared  to  earlier  picked  fruit  from  the  same  tree, 
it  was  found  that  the  earlier  fruit  was  almost  invariably  firmer  in 
texture. 

As  will  be  discussed  later,  shipping  quality  in  any  variety  seems 
to  be  rather  closely  associated  with  the  firmness  of  the  fruit  when 
picked.  Consequently,  the  earliest  fruit  on  the  tree  to  reach  a  given 
color  will  usualy  have  better  carrying  quality  than  the  later  ripen- 
ing fruit,  picked  in  the  same  color  stage.  These  results  indicate  that 
the  first  picking  of  any  variety  may  be  allowed  to  carry  more  color 
than  the  later  pickings,  and  have  equally  good  shipping  quality. 

Not  only  do  different  varieties  vary  in  the  firmness  of  the  flesh 
when  in  similar  maturity  stages,  but  the  rate  of  softening  also  varies 
greatly.  Late  varieties,  such  as  Giant  and  President,  soften  much 
more  slowly  than  such  varieties  as  Beauty  or  Climax.  The  average 
rate  of  softening  while  the  fruit  is  coloring  on  the  tree  usually  ranges 
from  %  to  1  pound  per  day. 

Increase  in  Soluble  Solids. — Since  hydrometer  tests  for  soluble 
solids  have  been  used  for  standardizing  picking  practices  for  citrus 
fruits  and  grapes,  and  such  tests  are  quickly  and  easily  made,  it  was 
thought  they  might  serve  as  a  measure  of  maturity  for  plums.  Juice 
was,  therefore,  extracted  from  each  sample  harvested  and  the  soluble 
solids  determined  by  means  of  Balling  scale  hydrometers.  The  results 
of  these  determinations  are  recorded  in  column  5,  table  2. 

Between  the  time  of  the  earliest  pickings  and  those  when  the  fruit 
is  of  practically  full  color  there  is  an  increase  of  from  15  to  25  per  cent 
in  the  quantity  of  soluble  solids.  This  increase  was  found  not  to  be 
gradual,  but  to  come  in  larger  part  after  the  fruit  is  considered  to  be 
too  mature  for  eastern  shipment.  The  increase  during  the  earlier 
ripening  stages  will  be  noted  to  be  from  almost  nothing  to  10  or  12 
per  cent.  Incidentally  samples  from  different  orchards  often  show 
greater  differences  when  in  similar  color  and  firmness  stages  than  is 
noted  between  two  maturity  stages.  The  increase  in  specific  gravity 
of  the  juice,  therefore,  is  not  sufficiently  closely  correlated  with 
maturity  and  carrying  quality  in  plums  to  be  of  value  as  an  index  for 
picking  maturity. 


Bul.  428] 


MATURITY    OF    SHIPPING    PLUMS 


11 


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UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


Dessert  quality 
(fruit  full  ripe) 

(15) 

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MATURITY   OF   SHIPPING   PLUMS 


13 


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20  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

Increase  in  Sugar  Content. — Dessert  quality  in  fruits  is  generally 
closely  associated  with  the  sugar  and  acid  content.  In  addition  to  the 
hydrometer  tests,  popularly  known  as  "sugar  tests"  but  which  only 
roughly  parallel  actual  sugar  content,  analyses  were  made  of  the 
actual  sugar  content  of  the  fruit  when  picked  at  various  color  stages. 
Methods  for  sugar  determinations  were  as  follows : 

Small  sectors  of  fruit  from  15  to  20  plums,  totaling  50  grams  in 
weight  were  boiled  5  minutes  with  125  c.c.  of  95  per  cent  alcohol  and 
1  gr.  CaCo3.  After  grinding  in  a  mortar,  sample  was  made  up  to 
500  c.c.  with  50  per  cent  alcohol,  and  extracted  with  daily  shaking 
for  seven  days.  The  alcohol  was  then  driven  from  a  sample  of  the 
supernatant  liquid  which  was  then  cleared  with  neutral  lead  acetate, 
centrifuged,  and  deleaded  with  sodium  oxalate.  Reducing  sugars  in 
this  solution  were  determined  by  the  Schaffer — Hartmann  method.7 

Inversion  of  the  non-reducing  sugars  was  accomplished  by  treating 
with  10  per  cent  citric  acid  in  a  boiling  water  bath  for  12  minutes,  and 
total  sugars  determined  after  inversion. 

The  data  on  total  sugar  content  of  the  fruit  at  time  of  picking  are 
shown  in  column  7,  table  2.  It  is  apparent  that  in  every  variety  there 
was  a  marked  increase  in  the  sugar  content  as  the  fruit  ripened  on  the 
tree.  This  continued  at  approximately  a  uniform  rate  during  the 
period  over  which  tests  were  made.  The  increase  in  total  sugar 
usually  averaged  from  10  to  15  per  cent  for  each  color  stage  or  picking 
condition  here  recorded.  These  data  are  in  accord  with  the  fact  that 
in  general  the  sugar  content  of  fruits  increases  rapidly  as  they  mature 
on  the  tree. 

There  was  not  a  close  relationship  between  the  hydrometer  test  of 
the  juice  and  the  actual  sugar  content  of  the  fruit.  With  most 
varieties  the  ratio  of  soluble  solids  to  sugars  is  approximately  that 
of  2  to  1.  From  the  data  presented  and  also  other  data,  considered 
too  fragmentary  to  include  in  table  2,  it  would  seem  that  this  ratio 
varies  slightly  as  the  fruit  ripens,  the  earlier  pickings  tending  toward 
a  wider  or  higher  ratio  between  soluble  solids  and  sugars  while  in  the 
better  colored  fruit  the  sugars  are  usually  slightly  more  than  half  of 
the  hydrometer  readings. 

The  analyses  for  reducing  sugars  are  given  in  column  9  of  table  2. 
These  sugars  are  relatively  stable  in  quantity  throughout  the  ripening 
period,  although  usually  increasing  slightly.  The  differences  between 
the  reducing  and  total  sugars  (column  7)  represent  the  sucrose  or  cane 
sugar  present.     This  shows  a  rather  marked  increase   between  the 


7  Shaffer,  P.  A.,  and  A.  F.  Hartmann.     The  iodometric  determination  of  copper 
and  its  use  in  sugar  analysis.    Jour  Biol.  Chem.  45:349-390.     1921. 


BUL.  428]  MATURITY   OF   SHIPPING   PLUMS  21 

different  color  stages,  usually  being  approximately  twice  as  high  in  the 
late  pickings  as  in  the  early  picked  fruit.  Thus  the  increase  in  sugar 
as  the  fruit  ripens  is  primarily  in  sucrose,  a  form  sweet  to  the  taste. 

The  proportion  of  sucrose  to  the  total  quantity  of  sugar  present 
varies  in  the  different  varieties,  Beauty,  Climax  and  Santa  Rosa  con- 
taining only  from  one-sixth  to  one-fourth  of  their  total  sugar  as 
sucrose,  while  in  most  of  the  other  varieties  tested  the  quantity 
increases  from  one-third  to  approximately  one-half  the  total. 

Decrease  in  Acid  Content. — Portions  of  the  same  fruits  used  for 
sugar  analysis  were  also  used  for  acid  determinations.  Acidity  in  the 
juice  was  determined  by  titrating  10  cc.  of  juice  with  N/10  NaOH, 
Methyl  red  proving  to  be  the  most  satisfactory  indicator.  Acidity 
was  calculated  as  malic  acid. 

In  addition  to  the  acidity  of  the  pressed  juice,  acid  in  extracted 
samples  of  the  pulp,  representing  proportional  quantities  of  skin,  and 
flesh  from  all  portions  of  the  fruit,  was  determined.  The  samples  were 
boiled  in  distilled  water,  and  make  up  to  volume,  and  preserved  with 
about  0.5  per  cent  toluene.  After  not  less  than  three  days  standing 
extraction,  with  daily  shaking,  the  acidity  of  the  supernatant  liquid 
was  determined  as  above. 

In  contrast  to  6  to  10  per  cent  of  sugar  in  plums  when  ripe,  they 
contain  only  0.5  per  cent  to  3  per  cent  of  acid.  The  ratio  of  acid  to 
sugar  is  variable,  both  between  varieties  and  within  the  same  variety. 
While  a  difference  of  1  or  2  per  cent  of  sugar  results  in  greater  sweet- 
ness, small  increases  in  the  amount  of  acid  influence  the  flavor  even 
more  markedly.  The  sweetest  varieties  of  plums  such  as  Gaviota  and 
Kelsey  contain  when  ripe  less  than  1  per  cent  of  acid,  whereas  the 
Diamond,  noted  for  its  acidity,  contains  2  per  cent  or  more. 

The  quantities  of  acid  in  the  juice  and  also  in  the  fruit  as  a  whole 
are  given  in  table  2,  columns  11  and  13,  respectively.  In  practically 
all  cases,  there  was  a  distinct  decrease  in  the  acidity  of  the  pressed-out 
juice  as  the  fruit  ripened  on  the  tree.  This  decrease  in  some  cases 
was  as  much  as  50  per  cent  of  the  acid  present.  The  greatest  pro- 
portionate decrease  occurred  in  Wickson,  Beauty,  and  Duarte,  while 
the  least  decrease  occurred  in  Diamond. 

A  slight  decrease  in  acidity  also  generally  occurred  when  the  fruit 
as  a  whole  was  analyzed,  although  the  decrease  was  much  less  than  in 
the  extracted  juice.  In  the  fruit  as  a  whole,  Santa  Rosa,  Wickson, 
Beauty,  Formosa  and  Climax  showed  only  slight  decrease  in  acidity 
with  advancing  maturity.  A  very  large  part  of  the  acidity  of  these 
varieties,  particularly  in  the  more  mature  fruit,  seems  to  be  localized 


22  UNIVERSITY   OF    CALIFORNIA — EXPERIMENT   STATION 

near  the  skin,  and  in  the  flesh  near  the  pit.  Juice  coming  from  the 
main  portion  of  the  flesh  contains  relatively  much  less  acid.  Con- 
sequently, fruit  of  these  varieties  tastes  sweet  and  pleasant  when  the 
juice  and  central  flesh  is  eaten,  but  very  sour  if  the  skin  and  the  flesh 
from  near  the  pit  is  eaten.  In  the  early  picked  lots,  there  was  rela- 
tively little  difference  in  the  acidity  of  the  pressed-out  juice  and  of 
the  fruit  as  a  whole.  As  the  fruit  of  the  Japanese  plums  matured 
on  the  tree,  less  of  the  total  acid  was  present  in  the  juice,  and  more 
in  the  skin  and  deep  lying  flesh.  This  more  rapid  loss  of  acid  in  the 
juice  continues  after  harvesting. 


CHANGES  IN  THE  FRUIT  FOLLOWING  PICKING 

Removal  from  the  tree  marks  the  cessation  of  growth  and  may  be 
said  to  limit  the  sugar  content  of  the  fruit.  Changes  in  color,  firmness 
and  acid  continue. 

Change  in  Color. — Plums,  unless  picked  previous  to  the  lightening 
of  the  green  color,  not  only  change  to  a  straw  or  yellow  after  being 
harvested  but  also  have  the  ability  in  the  absence  of  light  to  develop 
their  characteristic  red  or  blue  overcolor.  The  rate  of  this  develop- 
ment depends  largely,  if  not  entirely,  upon  the  temperature  to  which 
the  fruit  is  exposed.  Under  general  atmospheric  temperatures  at 
harvest  time,  perhaps  only  a  week  will  be  necessary  to  become  of  full 
color.  Under  the  temperature  usually  existing  in  the  top  of  refrig- 
erator cars  the  fruit  is  well  colored  at  the  time  of  its  arrival  on  the 
eastern  markets  while  that  loaded  in  the  bottom  of  the  car  has  changed 
color  but  little.  Fruit  picked  before  its  ground  color  begins  to  change 
and  carried  under  low  temperatures  usually  will  not  color  or  ripen 
satisfactorily. 

Firmness.— Softening  of  the  flesh  continues  following  picking. 
Most  varieties  of  plums,  testing  between  12  and  20  pounds  when  picked 
soften  to  1  or  2  pounds  when  in  best  eating  condition.  Again  this 
change  varies  greatly  with  differences  in  temperature,  as  will  be  dis- 
cussed under  rates  of  ripening  at  different  temperatures. 

Sugar  Content. — At  the  time  of  harvesting  for  commercial  ship- 
ment, plums  not  only  are  firm  and  possess  little  color,  but  have  little 
sweetness  or  flavor.  An  important  question,  therefore,  arises  as  to  the 
increase  in  sugar  content  after  picking.  Comparison  of  column  8 
with  column  7  of  table  2  will  show  that  on  the  average  there  is  prac- 
tically no  change  in  the  total  quantity  of  sugar  in  plums  between  the 
time  of  harvesting  and  the  full  ripe  stage.     In  the  Burbank  and 


BUL.  428]  MATURITY   OF   SHIPPING   PLUMS  23 

President  varieties,  there  was  a  slight  increase  in  sugars  during  this 
period,  but  in  all  other  varieties  there  was  not  a  significant  change  in 
sugar  content  following  picking.  There  also  was  no  significant  varia- 
tion in  the  form  of  the  sugar  as  the  fruit  ripened  following  picking, 
the  relative  proportions  of  sucrose  and  reducing  sugars  remaining 
approximately  the  same. 

Thus  the  sugar  content  of  the  fully  ripened  plums  is  determined 
practically  entirely  by  the  sugar  content  when  picked.  The  sweet 
taste  of  the  fully  ripe  fruit  as  compared  to  the  same  fruit  when  picked 
can  not,  therefore,  be  explained  by  an  increase  in  its  sugar  content. 

Acidity. — From  the  data  in  columns  11,  12,  13  and  14  of  table  2, 
giving  analyses  for  acid  in  the  pressed-out  juice  and  in  the  fruit  as 
a  whole  when  picked  and  when  fully  ripe,  it  must  be  concluded  that 
the  apparent  greater  sweetness  in  ripe  plums  is  in  reality  due  to  the 
relatively  large  decrease  in  the  percentage  of  acid  in  the  juice.  With 
most  of  the  Japanese  varieties  the  acid  in  the  pressed  juice  of  ripe 
fruit  was  only  from  one-fourth  to  one-half  as  much  as  in  similar  fruit 
when  picked.  Total  acidity  in  the  fruit  as  a  whole  (columns  13  and 
14)  did  not  show  a  marked  reduction  between  the  time  of  picking 
and  when  fully  ripe.  There  was  very  little  significant  variation  in  this 
regard. 

..  It  is  apparent  that  as  the  Japanese  plums  ripen  following  picking, 
the  acid  disappears  from  the  juice,  and  apparently  concentrates  in 
the  skin  region  and  near  the  pit.  When  eating  Japanese  plums  it  is 
noticeable  that  although  the  juice  has  a  pleasing,  sweet  taste,  the 
flesh,  particularly  near  the  skin  and  pit  is  very  sour  and  astringent. 

The  Diamond  variety,  belonging  to  the  species  Prunus  domestica 
or  European  plums,  showed  relatively  little  decrease  in  acidity,  either 
in  the  juice  or  in  the  fruit  as  a  whole,  during  the  interval  between 
harvest  and  the  full  ripe  condition. 

RELATION  OF  MATURITY  TO  DESSERT  QUALITY 

Maturity  of  plums  at  the  time  they  are  picked  has  a  most  important 
relation  to  their  dessert  quality.  The  sugar  content  is  higher,  the 
acidity  is  lower,  and  associated  with  these,  the  texture  and  aroma  are 
better  in  plums  which  are  tree  ripe  when  picked.  As  shown  in  table  2, 
column  15,  where  the  dessert  quality  of  fruit  picked  in  various  stages 
of  maturity  is  recorded,  the  early  pickings  were  generally  poor  to  fair 
in  quality,  while  the  later  pickings  were  good  to  very  good  or  excellent. 
Within  the  range  of  picking  maturity  possible  in  fruit  intended  for 
eastern  shipment,  the  more  nearly  mature  the  fruit  when  picked,  the 


24  UNIVERSITY   OF    CALIFORNIA — EXPERIMENT    STATION 

higher  the  dessert  quality  will  be.  There  is  relatively  little  change 
in  the  sugar  content  of  plums  following  picking,  and  the  quality  of 
the  ripened  product  is  largely  determined  by  the  maturity  at  picking 
time. 

Early  varieties  such  as  Beauty  and  Climax  are  sometimes  harvested 
exceedingly  green,  when  showing  only  slight  breaking  in  their  green 
color.  Such  fruit  not  only  has  a  very  low  sugar  content  and  a  cor- 
respondingly high  acid  content,  but  may  fail  to  color  and  soften 
properly.  The  flesh,  rather  than  becoming  tender  and  possessed  of  a 
pleasing  flavor,  becomes  tough  and  is  either  acrid  or  insipid.  Wrink- 
ling of  the  skin  and  general  wilting  of  the  fruit  are  also  of  frequent 
occurrence.  Such  fruit  is  not  only  unattractive  but  in  many  cases  is 
entirely  inedible. 

DETERMINATION  OF  SHIPPING  QUALITY 

The  shipping  quality,  or  the  way  plums  hold  up  under  actual  ship- 
ping and  commercial  conditions,  when  picked  in  various  stages  of 
maturity  was  determined  in  two  ways.  As  mentioned  above,  lots  of 
all  varieties  studied,  picked  in  different  maturity  stages,  were  shipped 
to  the  laboratory  and  held  under  temperature  conditions  which  closely 
approximated  the  average  condition  in  the  top  fruit  layers,  and  also 
in  the  lower  fruit  layers,  of  a  standard  refrigerator  car  while  in  transit 
from  California  to  the  Atlantic  seaboard. 

In  addition  to  this  laboratory  study,  one  of  the  writers  also 
cooperated  with  the  California  Fruit  Exchange  during  1924,  when 
cars  loaded  with  plums  including  special  test  crates  were  accompanied 
in  transit  on  a  test  trip.  Car  temperatures  were  taken,  and  the  condi- 
tion of  the  fruit  when  loaded  and  when  unloaded  from  various  portions 
of  the  car  was  carefully  noted. 

During  1925,  selected  crates  of  all  varieties  studied,  picked  in 
different  stages  of  maturity,  were  loaded  into  the  top  and  bottom 
layers  of  commercial  cars,  these  crates  being  equipped  with  recording 
thermometers,  packed  in  with  the  fruit.  These  crates  were  carefully 
inspected  in  New  York,  and  the  condition  of  the  fruit  upon  arrival 
recorded.  This  fruit  was  also  held  at  prevailing  New  York  tem- 
peratures following  unloading,  to  determine  its  holding  quality  under 
market  conditions. 

Finally,  during  the  season  of  1926,  the  various  commercial  lots  of 
fruit  loaded  into  twenty-five  cars  destined  for  New  York  City  were 
inspected,  and  the  packages  numbered  in  such  a  way  that  the  position 
which  any  package  had  occupied  in  the  car  could  be  determined  after 


BUL.  428]  MATURITY   OF   SHIPPING   PLUMS  25 

the  car  was  unloaded.  A  careful  record  was  made  of  the  condition  of 
arrival  of  these  commercial  lots,  whose  condition  at  loading  was  known. 
From  all  of  these  studies,  it  has  been  possible  to  determine  fairly 
accurately  the  carrying-  or  shipping  quality  of  the  different  varieties 
when  harvested  at  various  maturity  stages. 


HOLDING  TESTS,  AT  CAK  TEMPERATURES 

Under  present  transit  conditions,  approximately  12  days  is  required 
to  transport  fruit  from  California  to  the  Atlantic  seaboard.  During 
this  time,  fruit  in  a  car  is  held  at  varying  temperatures,  depend- 
ing upon  its  position  in  the  car,  and  also  upon  the  time  that  has  elapsed 
since  loading.  Results  of  a  number  of  tests,  details  of  which  will  be 
discussed  later,  show,  however,  that  the  average  temperature  in  the 
top  fruit  layer  of  a  carload  of  plums  loaded  warm  and  shipped  under 
standard  refrigeration  for  a  period  of  12  days  is  usually  about  53°  F. 
In  the  bottom  layer,  the  average  temperature  under  these  same  condi- 
tions will  average  about  40°  F.  In  the  layers  half  way  up  in  the  car, 
the  average  temperature  will  be  around  45  to  47°  F.  The  rate  of 
ripening  of  plums  picked  at  various  stages  of  maturity,  and  held  for  a 
12-day  period  at  52°  to  53°  F,  representing  top  of  load  temperatures, 
and  at  43°  F,  representing  the  lower  half  of  the  load  temperatures  has 
been  determined. 

The  condition  when  picked,  and  the  color,  firmness  and  market 
condition  after  12  days  at  these  temperatures,  are  recorded  in  table  3. 
Pressure  tests  at  the  end  of  six  days  are  also  included. 

Changes  in  Color. — From  the  data  in  table  3  it  is  apparent  that,  at 
temperatures  similar  to  those  prevailing  in  the  top  of  cars,  even  the 
early  picked  fruit  of  the  different  varieties  was  practically  full  char- 
acteristic color  at  the  end  of  12  days.  Slightly  straw  tipped  Beauty 
became  full  red  as  did  also  light  to  full  straw  Climax  and  pink  tipped 
Santa  Rosa.  Light  straw  Wickson  changed  to  yellow  and  blue  Diamond 
became  full  blue  during  this  period.  Fruit  picked  in  a  more  advanced 
color  stage  was  usually  of  full  color.  Santa  Rosa  picked  when  full 
light  red  becomes  dark  and  unattractive. 

At  43°  F  very  much  less  color  development  occurred.  Usually 
fruit  which  was  approximately  half  colored  when  picked  would  be 
full  light  color  of  characteristic  shade  when  removed  from  storage. 
All  varieties  showed  color  development  at  this  temperature,  but  much 
less  marked  than  at  52°  F.  If  the  fruit  in  a  car  is  loaded  at  a  uniform 
color  stage,  upon  arrival  the  top  layers  may  be  characteristic  color  for 
the  variety,  while  the  middle  and  lower  layers  will  show  far  less  color. 


26 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


31 


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MATURITY    OF    SHIPPING    PLUMS 


27 


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BUL.  428]  MATURITY   OF    SHIPPING   PLUMS  29 

Samples  of  much  of  this  fruit  was  also  held  at  35°  F.  The  data 
on  this  fruit  are  not  given,  but  relatively  little  increase  in  color 
occurred  during  a  12-day  period. 

Within  the  limits  of  picking  maturity  here  studied,  even  the 
greenest  lots  when  handled  under  temperatures  approximating  com- 
mercial shipping  conditions,  developed  characteristic  color  for  the 
variety.  Lots  of  fruit  only  partially  colored  when  removed  from  the 
lower  car  temperature  (43°  F)  usually  developed  full  color  in  two  or 
three  days  at  higher  temperature. 

Softening  During  Holding  Tests. — Samples  were  tested  both  at  the 
end  of  6  and  12  days  to  determine  the  rate  of  softening  under  the 
different  temperature  conditions.  Data  are  reported  in  columns  4,  5, 
8  and  9,  table  3. 

Plums,  in  common  with  most  other  deciduous  fruits,  soften  rela- 
tively rapidly  between  the  time  of  picking  and  the  time  they  are  eating 
ripe,  then  soften  slowly  until  overripe  and  wasty.  With  the  tester  here 
employed,  tests  of  two  pounds  or  under  usually  indicated  full  eating 
ripe  condition.  Fruit  testing  under  two  pounds  at  the  end  of  12  days 
would  almost  certainly  be  discounted  on  the  market  because  of  its 
condition.  Most  varieties  of  plums  here  studied  softened  about  as 
much  during  6  days  at  52°  F  as  they  did  in  12  days  at  43°  F.  This 
held  true  in  the  faster  softening  varieties,  including  Beauty,  Formosa, 
Climax,  Santa  Rosa  and  Giant.  Duarte,  Wickson,  and  President  were 
slightly  firmer  after  12  days  at  43°  F  than  after  6  days  at  52°  F.  For 
the  more  perishable  varieties,  it  is  apparent  that  softening  proceeds 
approximately  twice  as  rapidly  in  the  top  layers  of  fruit  in  cars  loaded 
warm  as  in  the  lower  portion  of  the  same  car. 

Some  of  the  earliest  picked  lots,  particularly  of  Wickson,  failed  to 
soften  and  ripen  properly,  shrivelling  instead.  Even  after  removing 
to  higher  temperatures,  this  fruit  remained  firm  and  was  practically 
valueless. 

Market  Condition  at  End  of  Twelve  Days. — Market  condition  at 
the  end  of  the  12  day  period,  columns  7  and  11,  table  3  has  been 
rated  primarily  on  the  color  and  firmness  of  the  fruit.  Fruit  of  full 
to  dark  color  for  the  variety,  or  which  was  soft  enough  to  be  in  prime 
eating  condition,  was  rated  as  slightly  ripe  or  too  ripe  for  best  market 
condition,  since  such  fruit  is  undesirable  for  successful  rehandling  and 
retailing.  Most  all  lots  held  at  43°  F  were  in  satisfactory  market 
condition  after  12  days,  except  some  of  the  very  ripe  pickings  of 
Beauty.  At  52°  F,  however,  the  riper  pickings  of  most  of  the  varieties 
studied  were  too  ripe  for  best  market  condition  after  12  days. 


30  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

EXPERIMENTAL  TEST  TRIP-SEASON,  1924 

In  order  to  secure  first-hand  information  on  the  carrying'  quality 
of  plums  and  other  early  fruit,  the  California  Fruit  Exchange  in 
June,  1924,  arranged  a  test  trip  of  six  cars  of  fruit,  destined  for  the 
Chicago  and  New  York  markets.  These  cars  originated  at  different 
shipping  points,  but  were  loaded  simultaneously  and  assembled  as  a 
unit  before  leaving  the  state.  Accompanying  the  cars  from  Roseville, 
California,  to  their  destination  were  representatives  of  the  Exchange, 
the  Pacific  Fruit  Express  Company,  the  California  State  Department 
of  Agriculture  and  the  University  of  California.  In  each  instance  one 
of  this  party  carefully  inspected  the  more  important  lots  of  fruit 
going  into  each  car  as  it  was  loaded  and  reinspected  these  lots  after 
their  arrival  at  destination.  All  cars  were  loaded,  handled  and  moved 
according  to  general  commercial  practice. 

In  addition  to  the  regular  commercial  fruit  lots,  loaded  without 
reference  to  their  position  in  the  car,  other  crates  were  included  where 
the  fruit  had  been  carefully  selected  as  to  different  degrees  of 
maturity,  varying  from  that  considered  slightly  too  immature  to  that 
somewhat  riper  than  is  generally  shipped.  These  crates  were  placed 
in  parts  of  the  car  known  to  represent  the  highest  and  lowest 
temperatures. 

Each  of  the  cars  under  test  was  equipped  with  electrical  resistance 
thermometers,  arranged  as  the  car  was  loaded,  so  that  air  and  fruit 
temperatures  could  be  ascertained  from  different  parts  of  the  car  at 
any  time  enroute  without  opening  the  door.  These  temperature  read- 
ings were  taken  at  four  to  six-hour  intervals  throughout  the  trip. 
The  average  temperatures  of  the  top  and  bottom  fruit  layers  are  shown 
graphically  in  figure  3.  The  lowest  temperatures  and  consequently 
those  most  favorable  to  retard  ripening,  were  at  the  bottom  of  the  load 
near  the  ice  bunkers,  while  the  highest  temperatures  were  found  to  be 
in  the  top  layers  half-way  between  the  door  and  the  end  of  the  car. 
Minimum  air  temperatures  in  the  first  position  were  from  40°  to  36°  F, 
while  those  in  the  second  position  were  from  55°  to  45°  F.  With  the 
temperature  of  the  fruit  75°-80°  F  when  loaded  and  an  outside  tem- 
perature of  80°-90°  F,  each  day,  fruit  in  the  cooler  portions  of  the 
cars  cooled  to  approximately  45°  F  within  12  hours  after  loading, 
while  it  required  6  to  7  days,  or  the  running  time  to  Omaha,  for  the 
fruit  in  the  warmer  portions  of  the  cars  to  cool  to  50°  F.  The  tem- 
perature of  the  fruit  throughout  the  top  layers  averaged  10°  to  16°  F 
above  that  in  the  bottom  layers,  this  difference  being  greatest  during 
the  first  three  or  four  days  in  transit. 


Bul.  428] 


MATURITY    OF    SHIPPING    PLUMS 


31 


Table  4  shows  the  condition  of  Climax  and  Santa  Rosa  plums 
shipped  in  the  car  from  Auburn,  California,  to  Chicago,  Illinois. 
While  this  car  arrived  at  destination  for  sale  on  June  27,  the  seventh 
morning  after  leaving  California,  the  fruit  was  not  unloaded  until 
the  night  of  the  29th,  making  the  holding  period  correspond  more 
closely  to  New  York  than  to  Chicago  delivery. 


TABLE  4 

Eipening  of   Test  Crates   of   Climax   and   Santa  Eosa  Plums   in   Transit. 

Auburn,  California,  to  Chicago,  Illinois.     June  19-30,  1924 


Variety 

Condition  when  loaded 

Position  in  car 

Condition  on  arrival 

Bottom,  tier  1 

Bottom,  tier  5 

Bottom,  tier  10.... 
Top,  tier  1  

Straw  to  red  tip;  firm 
to  springy. 

\i  to  %  red;  springy 

M  to  %  red  color;  very 
firm. 

M  to  full  red  color; 
firm. 

Full  to  dark  red ;  firm 
ripe. 

Straw  to  red  tip  to  x/i  red;  firm  to  springy. 
Straw  to  red  tip  to  \i  red;  firm  to  springy. 

Top,  tier  5 

Top,  tier  10 

Red;  firm  to  springy. 

Bottom,  tier  1 

Bottom,  tier  5 

Bottom,  tier  10.... 
Top,  tier  1 

Straw  to  Yz  red;  firm  to  springy. 
Straw  to  %  red;  firm  to  springy. 

Top,  tier  10 

Bottom,  tier  1 

Bottom,  tier  5 

Bottom,  tier  10.... 
Top,  tier  1 

%  to  full  red;  firm  to  springy. 

Top,  tier  5 

Top,  tier  10 

Santa  Rosa 

Bottom,  tier  1 

Bottom,  tier  5 

Bottom,  tier  10.... 
Top,  tier  1 

%  red  to  full  color;  firm. 

Full  to  dark  red;  firm  to  springy. 

Full  red;  firm  to  springy. 

Full  to  dark  red;  firm  to  springy. 

Top,  tier  10 

No  fruit. 

Santa  Rosa 

Bottom,  tier  1 

Bottom,  tier  5 

Bottom,  tier  10.... 
Top,  tier  1 

Full  to  dark  red;  firm  to  springy. 
Full  dark  red;  firm  to  springy. 
Dark  red;  firm  to  springy. 
Full  dark  red;  firm  to  springy. 

Top,  tier  5 

Full  dark  red;  firm  to  springy. 

Top,  tier  10  .... 

Santa  Rosa 

Bottom,  tier  1 

Bottom,  tier  5 

Bottom,  tier  10.... 
Top,  tier  1 

Full  to  dark  red;  springy  to  slightly  soft. 
Full  to  dark  red;  springy  to  slightly  soft. 
Dark  red;  soft. 

Top,  tier  5 

Dark  red;  springy  to  soft. 

Top,  tier  10  .. 

32 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


While  all  of  this  fruit  arrived  in  fair  condition,  the  riper  pickings 
when  loaded  in  the  top  of  the  car  arrived  too  ripe  to  be  well  received 
by  the  trade.  The  first  degree  of  maturity  of  Climax  was  rated  "good 
market  condition"  while  the  second  picking  loaded  in  the  top  of  the 
car  was  slightly  riper  than  desirable  and  the  third  lot,  picked  when 
%  to  %  red,  was  overripe.  Twelve  hours  after  unloading  from  the 
car,  the  second  lot  contained  a  number  of  soft  plums,  while  the  fruit 
of  the  third  picking  was  all  soft  ripe.  Santa  Rosa  was,  on  the  whole, 
considerably  riper  than  Climax.  Fruit  picked  in  the  second  stage  of 
maturity,  %  to  full  color,  and  carried  in  the  top  of  the  car  arrived  a 
dark  red  color  and  ready  for  immediate  use.  The  mature  picking 
showed  half  the  specimens  soft  within  12  hours  after  unloading.  As 
will  be  observed  from  the  table,  while  the  different  maturity  stages 
showed  differences  in  color  upon  arrival,  and  also  the  fruit  nearest  the 
end  of  the  cars  was  firmer  than  that  carried  nearer  the  center,  yet  the 
greatest  difference  was  between  the  bottom  and  the  top  of  the  load, 
corresponding  to  the  highest  and  lowest  temperatures  in  the  car. 

In  addition  to  these  placed  crates,  inspections  were  also  made  on 
regular  lots  carried  in  different  positions  in  the  car.  Inspections  of 
these  lots  served  to  show  the  general  average  shipping  quality  of 
several  additional  varieties  carried  under  the  same  conditions  as  the 
above  test  crates.    The  data  are  given  in  table  5. 

TABLE  5 

Eipening  of  Commercial  Lots  of  Plums  in  Transit.    Auburn,  California,  to 

Chicago,  Illinois,  June  19-30,  1924 


Variety 

Condition  when  loaded 

Condition  upon  arrival 

Straw,  red  tip  to  %  light  red;  springy 

Yellow  to  red,  mostly  lAto%  red;  springy.. 

Green  to  straw,  very  firm 

Straw  to  yellow,  springy 

Shiro 

Shiro 

Yellow-red  to  red;  springy  to  soft,  full  ripe. 

\i  to  %A  color;  firm  to  springy .' 

%  to  full  color,  a  few  dark  purple;  firm  to 
springy. 

Tragedy 

Full  tojiark^purple,  soft. 

As  with  the  Climax  and  Santa  Rosa  test  crates,  these  varieties, 
aside  from  the  early  picking  of  Shiro,  were  ready  for  immediate  use 
upon  arrival  and  would  stand  little  rehandling. 


BUL.  428]  MATURITY   OF   SHIPPING   PLUMS  33 


LOT  SHIPMENTS,  1926 

During  the  season  of  1926,  in  cooperation  with  the  different  ship- 
ping associations  of  Placer  County,  a  careful  study  was  made  of  the 
maturity  and  condition  of  arrival  of  commercial  shipments.  Approxi- 
mately 100  lots  of  ten  varieties  were  inspected  at  the  time  of  loading 
in  the  cars  and  the  packages  numbered  in  such  a  manner  that  the 
position  which  any  crate  occupied  in  the  car  could  be  ascertained  after 
the  fruit  was  unloaded.  With  the  shipping  point  inspection  data 
recorded  on  one-half  of  a  printed  form,  this  copy  was  forwarded  to 
the  United  States  Department  of  Agriculture  representative  in  New 
York  who  completed  the  record  after  making  his  observations.  On 
the  auction  market,  to  which  all  the  fruit  was  consigned,  it  was  not 
possible  to  examine  the  same  crates  as  previously  inspected,  yet  rep- 
resentative crates  of  each  grower's  lot  were  opened  and  the  general 
average  condition  of  the  shipment  as  a  whole  recorded.  The  results 
of  the  data  secured,  shown  in  table  6,  give  an  excellent  idea  as  to  the 
maturity  of  the  fruit  shipped  during  the  season  and  its  carrying 
quality.  It  also  furnished  a  further  check  upon  the  holding  tests 
discussed  above. 

From  table  6  the  differences  in  carrying  quality  of  the  fruit  loaded 
in  different  positions  of  the  car  is  further  emphasized.  Averaging  all 
varieties  shipped  throughout  the  season  most  of  the  shipments  arrived 
in  firm  condition.  However,  with  only  the  earliest  varieties  did  any 
of  the  fruit,  even  in  the  bottom  of  cars,  arrive  "slightly  green," 
whereas  a  rather  too  large  percentage  of  other  varieties,  particularly 
Santa  Rosa,  Wickson  and  Giant,  arrived  "slightly  ripe"  to  "ripe." 
Under  variable  temperatures  it  is  impossible  to  have  the  load  arrive 
100  per  cent  "firm"  without  grading  the  fruit  as  to  maturity  when 
loading.  As  this  seems  impractical  one  can  only  endeavor  to  harvest 
the  variety  at  a  stage  of  maturity  which  will  carry  with  the  minimum 
percentage  of  loss,  either  from  over  or  under  maturity.  During  the 
season  of  1926  it  would  seem  that  the  earlier  varieties  were  shipped 
in  about  their  most  satisfactory  stage  but  that  the  later  varieties  were 
in  some  instances  allowed  to  become  slightly  too  ripe  for  best  shipping. 


RELATION  OF  MATUEITY  TO  CARRYING  QUALITY 

From  the  above  discussion,  it  is  apparent  that  within  limits,  the 
earlier  the  fruit  is  picked  the  better  the  carrying  quality  will  be.  Fruit 
picked  before  it  is  beginning  to  show  a  color  change  will  usually 
shrivel  and  fail  to  ripen  properly,  but  once  the  color   change  has 


34 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


TABLE   6 
Shipping  Maturity  and  Condition  of  Arrival.     Placer  County  Plums,  1926* 


Variety  and  description 
when  shipped 


Beauty: 
60%  straw  tip. 
32%  pink  tip. 
8%  y  color. 

Climax: 
62%  green  with  straw 

tip. 
38%  straw  with  pink  tip. 

Formosa: 

10%  green  to  straw  tip. 

80%  full  straw. 

10%  pink  tip  to  y  pink. 

Santa  Rosa: 
21%  V2  to  H  red. 
79%  full  red. 


Wickson: 
22%  green  to  straw  tip. 
24%  light  straw. 
28%  full  straw  to  yellow 

tip. 
26%  Y2  yellow. 

Duarte: 
20%  slight  red  color. 
45%  H  color. 
35%  V3  full  color. 

Diamond: 

12%  slight  purple 

30%  y  purple. 

58%  M  to  full  purple. 


Burbank: 
70%  straw  tip  to  straw. 
10%  yellow. 
15%  pink  tip. 
5%  y  to  H  red. 

Giant: 
25%  slight  pink. 
40%  %  to  full  light  red. 
35%  full  red. 

Grand  Duke: 
5%  y  color. 

55%  %  to  full  light  color 
40%  full  color. 


Position  in  car 


Top  third 

Middle 

Bottom  third. 


Top  third 

Middle 

Bottom  third.. 


Top  third 

Middle 

Bottom  third. 


Top  third 

Middle 

Bottom  third.. 


Top  third 

Middle 

Bottom  third. 


Top  third 

Middle 

Bottom  third. 


Top  third 

Middle 

Bottom  third.. 


Top  third 

Middle 

Bottom  third. 


Top  third 

Middle 

Bottom  third. 


Top  third 

Middle 

Bottom  third.. 


Condition  on  arrival,  percentage 


Ripe 


Slightly 
ripe 


(Slightly 
Firm       green 


53 
84 
50 

38 
55 
100 


70 


95 


65 

85 
100 


85 
100 


Remarks 


Slight  pink  tip  carried 
well.  Fruit  y  pink 
or  more  in  top  of  car 
full  ripe. 

Fruit  carried  well. 
Most  crates  arrived 
'  firm." 


Fruit  shipped  with 
pink  tip  arrived 
slightly  overripe  in 
top  of  car. 

Most  of  fruit  in  top 
half  of  cars  arrived 
too  ripe  for  best  re- 
turns. 

Top  of  car  badly  over- 
ripe. 


Fruit  picked  with  full 
color  arrived  too  ripe, 
bruising  bad. 


Fruit  of  full  color  too 
ripe  for  top  of  car. 
6%  of  fruit  showing 
decay. 

For  best  carrying  qual- 
ity fruit  should  not 
be  past  a  good  straw 
color. 


Plums  more  than  light 
red  carried  satisfac- 
torily only  in  the 
lower  half  of  the  car. 

Most  of  fruit  arrived 
in  very  satisfactory 
condition. 


*  The  percentage  figures  refer  to  crates  or  other  packages,  rather  than  to  the  fruit  in  the  packages. 
Thus  a  variety  that  was  50  per  cent  pink  tip  is  one  that  showed  the  fruit  predominately  pink  tipped 
in  half  the  packages  inspected.  If  20  per  cent  of  the  top  third  is  reported  ripe,  it  means  that  20  per 
cent  of  the  packages  in  that  position  showed  predominately  a  ripe  condition. 


BUL.  428]  MATURITY    OF   SHIPPING   PLUMS  35 

definitely  begun,  the  riper  the  plums  when  picked  the  shorter  time 
they  will  hold  up  under  the  present  temperature  conditions  prevailing 
in  refrigerator  cars. 


TIME    OF    HARVESTING 

From  the  foregoing  discussion,  it  is  evident  that  two  main  factors 
must  be  considered  in  determining  at  what  stage  to  harvest  any  plum 
variety.  Within  limits,  the  greener  the  fruit  when  harvested  the 
better  will  be  its  carrying  quality.  On  the  other  hand,  the  riper  the 
fruit  when  harvested  the  better  will  be  its  dessert  quality  when  it 
reaches  the  hands  of  the  consumer. 

It  is  first  of  all  essential  that  the  fruit  be  delivered  to  the  market 
and  to  the  retailer  in  good,  firm,  attractive  condition.  The  finest 
flavored  fruit  cannot  be  sold  at  a  profit  if  it  is  overripe,  decayed  and 
wasty  on  the  retail  stands.  On  the  other  hand,  a  permanent  industry 
requires  that  fruit  of  the  highest  possible  dessert  quality  be  secured. 
Consequently,  the  questions  of  plum  maturity  are  (1)  "How  ripe 
can  the  different  plum  varieties  become,  prior  to  harvest,  and  still  be 
delivered  to  market  in  good  condition"  and  (2)  "What  is  the  earliest 
stage  at  which  these  varieties  can  be  harvested,  and  still  secure  fair  to 
good  dessert  quality. ' '  The  answer  to  the  first  of  these  questions  will 
vary  largely  with  the  temperatures  in  transit  and  the  handling 
conditions  to  which  the  fruit  is  exposed. 


TESTS  FOE  TIME  OF  PICKING 

The  firmness  of  the  fruit  when  picked  has  proved  to  be  most  closely 
associated,  of  any  of  the  factors  studied,  with  the  way  the  fruit  will 
hold  up  under  transit  conditions.  The  firmness  of  the  fruit,  or  the 
pressure  test,  is  a  more  accurate  index  to  carrying  quality  than  color, 
since  the  earliest  fruits  to  reach  a  certain  color  stage  are  almost 
invariably  firmer  and  of  better  carrying  quality  than  the  later  ripening 
plums  from  the  same  trees,  and  picked  in  the  same  color  stage. 

If  the  fact  that  late-coloring  plums  are  usually  softer  is  considered, 
color  becomes  a  fairly  safe  guide  to  picking  maturity  for  plums  of  the 
varieties  here  studied.  The  pressure  test  however  should  prove  of 
value  in  connection  with  color,  in  establishing  the  condition  of  fruit 
in  different  orchards  and  in  different  seasons.  Both  color  and  pressure 
tests  are  given  in  table  7,  where  recommended  picking  conditions  are 
summarized. 


36  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


PICKING  EECOMMENDATIONS 

Fruit  Shipped  Standard  Refrigeration. — Since  the  highest  quality 
in  the  plum  varieties  here  studied  is  secured  by  late  picking,  it  follows 
that  the  maturity  desirable  when  harvested  will  vary  with  the  tem- 
perature conditions  and  time  of  the  fruit  in  transit.  As  was  shown 
earlier,  the  average  temperature  in  the  top  layers  of  fruit  in  a  standard 
refrigerator  car  loaded  with  hot  fruit  and  12  days  in  transit  is  near 
53°  F.  Temperature  averages  in  the  test  trip  referred  to  earlier,  and 
in  several  cars  equipped  with  recording  thermometers  packed  in  the 
crates  of  fruit  loaded  in  the  top  layer  have  shown  the  top  layer  tem- 
peratures to  be  about  as  follows,  with  fruit  loaded  at  75°  to  80°  F. 

First  day  70° 

Second  day 60° 

Third  day  56° 

Fourth  day  54° 

Fifth  day  52° 

Sixth  day 50° 

Seventh  day  46-48° 


If  the  fruit  is  shipped  to  markets  closer  than  the  Atlantic  seaboard, 
average  temperatures  in  the  top  of  the  load  are  even  higher,  since 
during  the  first  few  days  the  fruit  is  relatively  warm.  For  example, 
during  a  six-day  trip,  the  top  temperatures  will  average  57°  F.  Thus 
even  for  relatively  nearby  markets,  the  fruit  must  be  firm  when  picked. 

The  top  layer  temperatures  represent  the  most  unfavorable  condi- 
tions in  the  car,  but  these  conditions,  rather  than  the  average,  must 
be  considered  in  determining  when  to  pick  the  fruit.  Fruit  picked  in 
condition  to  carry  satisfactorily  in  the  top  layers  of  the  car  will  be 
in  excellent  condition  upon  arrival  if  loaded  in  lower  portions  of 
the  car. 

Recommendations  in  table  7,  under  the  heading  "when  shipped 
standard  refrigeration"  are  based  on  three  seasons  holding  and  ship- 
ping tests.  The  color  and  firmness  range  recommended  include  as  the 
earliest  color  stage  and  firmest  fruit,  the  earliest  condition  in  which 
the  fruit  may  be  harvested  and  ripen  with  good  appearance  and  fair 
flavor.  The  greatest  amount  of  color  and  lowest  pressure  test  figure 
represents  about  the  most  mature  picking  stage  feasible  if  the  fruit  is 
loaded  in  the  top  of  the  car,  and  carried  under  the  conditions  outlined 
above.  If  fruit  is  riper  than  here  recommended,  it  is  likely  to  arrive 
on  the  eastern  markets  full  dark  color  and  soft  in  the  top  layers  and 
be  discounted  in  price. 


Bul.  428] 


MATURITY    OF    SHIPPING   PLUMS 


37 


TABLE  7 

Becommended  Picking  Condition  for  Plums.     Based  Upon  Temperatures  in 

Transit  in  the  Upper  Half  of  Refrigerator  Cars 


When  shipped  standard  refrigeration 

When  precooled  and  shipped 

Variety- 

Color  range 

Pressure 
test  range 
(pounds) 

Color  range 

Pressure 
test  range 
(pounds) 

CD 

(2) 

(3) 

(4) 

(5) 

13-  9 
13-  9 
18-13 
18-12 
20-14 

15-12 
15-11 
20-15 
16-11 
16-11 

8-  6 

8-  6 

12-  8 

M  to  full  light  red    .. 

12-  9 

Straw  tip  to  full  straw  with  slight 
red. 

13-  8 

11-  8 

Full  light  to  medium  red 

Full  light  to  medium  blue 

Full  light  red 

10-  8 

14-10 

}4to%  light  red 

10-  8 

Yz  to  full  light  blue  

Full  blue 

10-  7 

Fruit  picked  riper  than  the  condition  described  in  the  first  section 
of  table  7  can  be  carried  successfully  if  loaded  in  the  bottom  of  the 
car.  Where  therefore,  certain  lots  are  known  to  be  relatively  ripe 
they  will  carry  much  better  in  the  bottom  half  of  the  car,  than  if 
loaded  in  the  upper  layers. 

Fruit  Precooled. — Where  it  is  possible  to  thoroughly  cool  fruit 
prior  to  loading  or  shipment,  temperature  conditions  in  transit  will 
be  quite  different.  If  the  fruit  is  cooled  to  40°  F  prior  to  loading, 
it  will  from  the  start  of  the  period  in  transit,  be  carried  at  the 
lowest  temperature  at  which  the  ice  in  the  car  will  hold  it.  The  fruit 
in  the  top  layer  will  warm  up  to  from  46°  to  48°  F  and  carry  at 
that  temperature  while  in  transit.  The  top  layer  temperatures  in 
thoroughly  precooled  cars  should  not  average  above  45°  to  46°  F. 

Under  these  temperature  conditions,  fruit  somewhat  more  mature 
than  recommended  for  non-precooled  cars  can  be  shipped.  Columns 
4  and  5  of  table  7  give  recommendations  for  the  ripest  fruit  that  can 
be  handled  with  safety  under  such  conditions. 

When  precooling  is  not  available,  somewhat  faster  cooling  can  be 
secured  by  adding  2  to  3  per  cent  of  salt  to  the  ice.  Temperatures 
prevailing  in  such  a  car  are  shown  in  figure  4.  A  comparison  of 
figures  3  and  4  shows  that  cooling  was  somewhat  more  rapid  when  the 
salt-ice  was  used.  Temperatures  in  the  top  layer  of  the  car  shown  in 
figure  4  averaged  slightly  over  2°  F  lower  than  in  the  car  shown  in 
figure  3. 


sS« 


*ht  i 

in  1 1  n 


t  t 

i   I 


* 


t 


SXf 

85° 
75" 

1 

1    1 

A 

^^r-r  «*  bunk cks  at  oesrfhi.  V$tt* 
AfTT    consumed    EH*owrc  JtZ/5C~ 

\ 

A 

-Oaiside   Air  Tempera-f-ures 

A 

1 

/\ 

A 

1 

/  \ 

70' 

59 
50* 
45' 

• 

J\.      1 

i  \ 

'   ~*1k 

V 

r     ' 

^ 

- — 

fcw      _j 

^■Average  Temperature  of  Fruir  in  Upper-  Layer 

» 

^»— «— 

<~-* 

*^ 

• 

v, 

" 

— ■-,_     -- 

— — ' 

•fc«»" 

-»_-' 

-•« 

'••-~- 

'«.  Temp,  af 

Fruit 

/>?  Lovjer  Louer 

••••»•• 

— .— . 

,.— — 

30 

AJt  Rn  AJl!  KM.  Art'Prt.  Art!  PM.  Art.  P.M.  Art'.  P.M.  Art' PM.  A.M. '  PM.  Art' P.M.  Art!  P.M.  Art!  P.M.  Art!  PM. 
*/l9   ,     e/ZO  ,     *fa   .     */2z  ,    6/Z3   ,     */24  ,     Cfes  ,    £JZG  ,     6/Z7  ,    */Z6  ,    6/29   ,     6/SO 

Art!  KM 

Record  of  Temperatures  /n  P.ff.  Refrigerator  Car  No-7033. 

Plums   and   Peaches  -  Loom/'s ,  Calif,   -fo  New  York ,  N.  Y. 

945  Standard  Crates  Plums  &  Z6*  70  Boxes  Peaches  (BZ/'/z*.  Total  hit  Z607S* 

Loaded   10  High  -  o~  Wide ,  Zl  S-facks  on    Sfandard    floor    Racks  . 

Standard    JSefrigera-f/or? 

Route!   S.R  -  U.P.    -  I.H.B.    -  Brie 


Fig.  3. — Average  fruit  temperatures  under  standard  refrigeration. 
California,  to  New  York,  N.  Y. 


Kosevilie, 


Record  of  Temperatures   in  P.f.L\.    Pefrt'gerafor   Car  Alo.7193 . 
Plums  and  Figs  -  Newcastle,  Calif,     to   New   York,  N.  Y. 
3lCrai*  Has  e  9*,  63  5t'd.  Boxes  Plum's  9  Z/*,  950  5td  Craies  Plums  0  Z6*   Total  W+.  Z6305* 
Loaded  IO  High-  5  Wide  -  Zl  Slacks ,   on  Standard   floor    Packs. 
Standard  Refrigeration  and   Salt. 
Route  :  S.P.  -U.P- I.C.  -I.H.B  ~  Erie 


Fig.  4. — Average  fruit  temperatures  in  refrigerator  car.    Newcastle,  California,  to 
New  York,  N.  Y.     Standard  refrigeration  with  addition  of  salt. 


BUL.  428]  MATURITY   OF   SHIPPING   PLUMS  39 

The  necessity  of  getting"  the  fruit  under  refrigeration  as  early  as 
possible  should  also  be  emphasized.  Such  fruits  as  plums  ripen  almost 
twice  as  rapidly  at  52°  as  at  43°  F.  At  70°  F  or  above,  the  tem- 
peratures usually  prevailing  during  the  harvest  season,  ripening  pro- 
ceeds with  great  rapidity.  The  more  quickly  the  fruit  can  be  cooled 
following  picking,  the  quicker  ripening  will  be  retarded,  and  the  better 
the  condition  of  the  fruit  upon  arrival  in  eastern  markets. 

SUMMARY 

CHANGES  IN  FEUIT  BEFOEE  HARVESTING 

Increase  in  Size. — Shipping  plums  continue  to  increase  in  size 
until  they  become  full  colored,  the  rate  of  increase  becoming  somewhat 
less  as  the  fruit  approaches  full  ripeness.  At  the  time  plums  are 
usually  harvested  for  eastern  shipment,  they  are  increasing  in  volume 
from  1.5  to  2  per  cent  daily.  Picking  earlier  than  is  necessary  for 
proper  carrying  quality  results  in  smaller  fruit  and  considerable  loss 
in  tonnage. 

Increase  in  Color. — Changes  in  color  are  definitely  associated  with 
ripening.  Coloring  on  the  tree  is  influenced  by  the  exposure  to  sun- 
light, those  fruits  on  the  upper  or  outer  branches  coloring  earlier  than 
those  produced  on  the  lower  or  inner  branches.  In  some  instances  the 
latter  fruits  on  a  given  date  are  as  soft  as  those  of  higher  color. 

Softening  of  the  Flesh. — Softening  of  the  flesh  occurs  as  plums 
mature  on  the  tree.  Measured  by  a  mechanical  tester,  the  firmness 
of  the  flesh  gives  an  accurate  index  of  the  maturity  and  shipping 
quality  of  plums. 

Increase  in  Soluble  Solids. — Hydrometer  readings  on  the  juice  of 
plums  show  a  continuous  increase  in  soluble  solids  from  the  first  color 
changes  to  full  maturity  of  the  fruit.  The  amount  of  this  increase 
during  the  possible  harvest  season  is  not  sufficient  for  hydrometer 
readings  to  serve  as  an  index  to  picking  maturity. 

Increase  in  Sugar  Content. — Sugar  content  increases  as  long  as  the 
fruit  remains  on  the  tree.  Actual  sugar  analyses  show  approximately 
half  as  much  sugar  present  as  the  total  soluble  solids  indicated  by  the 
Balling  hydrometer. 

Changes  in  Acid  Content. — Plums  contain  from  1  to  3  per  cent  of 
acid  when  picked,  this  amount  gradually  decreasing  with  the  latter 
pickings.  In  early  picked  fruit,  the  expressed  juice  and  the  fruit  as 
a  whole  are  approximately  equal  in  acidity.  In  later  pickings  there  is 
much  less  acid  in  the  expressed  juice. 


40  UNIVERSITY   OF    CALIFORNIA — EXPERIMENT   STATION 


CHANGES  IN  FKUIT  AFTEK  HABVESTING 

Color. — Unless  picked  exceedingly  immature,  plums  are  able  to  con- 
tinue their  color  development  after  being  harvested  and  in  the  absence 
of  light.  This  change  is  influenced  greatly  by  temperature,  little  color 
development  occurring  when  the  fruits  are  held  below  36°  F. 

Softening. — After  picking,  plums  rapidly  lose  their  firmness.  The 
rate  of  softening  is  largely  controlled  by  temperature.  At  52°  F  most 
varieties  will  soften  approximately  twice  as  rapidly  as  at  43°  F.  When 
full  eating  ripe  they  will  test  from  one  to  two  pounds. 

Soluble  Solids. — There  is  practically  no  increase  in  soluble  solids 
after  picking. 

Sugar  Content. — While  a  few  of  the  larger,  later-maturing  and 
more  meaty  types  of  plums  apparently  make  a  slight  gain  in  their 
total  quantity  of  sugar  after  picking,  most  varieties  show  no  increase 
after  being  removed  from  the  tree. 

Acidity. — The  acid  content  of  the  expressed  juice  of  plums 
decreases  greatly  after  harvesting.  The  fruit  as  a  whole  shows  only 
a  slight  decrease  in  acidity  during  this  ripening  period. 


DETEKMINATION  OF  SHIPPING  MATUEITY 

Car  Temperatures  and  Rates  of  Ripening  in  Transit. — Tempera- 
ture records  taken  in  transit  show  the  temperature  in  cars  of  plums 
under  standard  refrigeration  to  average  about  40°  F  on  the  floor  of 
the  car  and  about  53°  F  in  the  top  fruit  layers,  during  a  12-day  period. 
With  an  outside  air  temperature  of  80°-90°  F  each  day  and  the  fruit 
registering  75°-80°  F  when  loaded,  that  placed  in  the  coolest  part  of 
the  car,  was  cooled  to  45°  F  in  12  hours,  while  approximately  6  days 
was  required  to  lower  the  temperature  in  the  top  layers  to  50°  F. 
Plums  loaded  in  the  top  layers  ripen  far  more  while  in  transit  than 
does  fruit  in  the  lower  parts  of  the  car. 

Inspections  of  Commercial  Lots. — Shipping  point  inspection  of 
commercial  shipments  and  reinspection  at  destination  indicated  that 
most  plum  varieties  are  being  picked  in  as  mature  a  condition  as  is 
feasible  under  present  handling  methods. 

Dessert  Quality. — The  more  mature  plums  are  allowed  to  become 
before  harvesting  the  higher  the  sugar  content  and  the  better  their 
flavor.  Fruit  picked  before  showing  some  change  in  the  green  ground 
color  of  the  skin  may  fail  to  ripen  or  soften  properly. 


BUL.  428]  MATURITY   OF   SHIPPING   PLUMS  41 

Shipping  Quality. — Within  limits,  the  earlier  picked  fruits  carry 
better  under  prevailing  car  temperatures  than  more  mature  fruit. 
Fruit  must  be  picked  sufficiently  firm  to  carry  to  market  in  good 
condition. 

Picking  Indexes. — The  firmness  of  the  flesh  and  color  of  the  fruit 
furnish  the  most  satisfactory  indexes  of  picking  maturity.  Detailed 
recommendations  are  given  (table  7)  for  picking  condition  of  different 
varieties,  depending  upon  handling  methods. 

The  Possibility  of  More  Mature  Picking. — Under  present  methods 
of  handling  in  transit,  most  plum  varieties  are  picked  in  as  mature  a 
condition  as  they  can  be  satisfactorily  delivered.  Where  precooling 
is  feasible  somewhat  riper  and  consequently  larger  and  better  quality 
fruit  may  be  shipped  than  is  possible  under  standard  refrigeration. 
The  addition  of  2  to  3  per  cent  of  rock  salt  to  the  ice  in  the  car  bunkers 
accomplished  slightly  quicker  cooling. 


ACKNOWLEDGMENTS 

In  connection  with  this  work,  the  writers  feel  greatly  indebted  for 
assistance  received.  Mr.  J.  A.  Teagarden,  chairman,  and  members  of 
the  Placer  County  Fruit  Growers  Committee  on  Fruit  Maturity, 
helped  in  many  ways.  The  investigation  and  the  results  secured  are  in 
a  large  measure  due  to  their  enthusiastic  efforts. 

The  California  Fruit  Exchange  invited  the  cooperation  of  the 
University  of  California  and  the  United  States  Department  of  Agri- 
culture in  the  car  test  trip  and  allowed  the  use  of  data  secured  from 
this  test.  The  shipping  organizations  of  Placer  County  cooperated 
heartily  in  allowing  inspection  records  to  be  made  on  their  commercial 
shipments,  both  at  loading  point  and  in  New  York. 

Chemical  analyses  reported  in  this  bulletin  were  made  by  H.  W. 
Allinger  and  F.  G.  Anderssen.  H.  S.  Henika  assisted  in  the  field  work 
throughout  the  investigation.  Field  and  laboratory  assistance  was 
also  received  at  various  times  from  J«  L.  Stahl,  G.  S.  Pine  and  N.  E. 
Berry. 


STATION  PUBLICATIONS   AVAILABLE   FOR  FREE   DISTRIBUTION 


No. 

253.   Irrigation   and   Soil   Conditions  in  the 
Sierra   Nevada   Foothills,    California. 

261.  Melaxuma    of    the    Walnut,    "Juglans 

regia." 

262.  Citrus   Diseases  of   Florida   and   Cuba 

Compared  with  Those  of  California. 

263.  Size   Grades   for  Ripe   Olives. 

268.   Growing  and  Grafting  Olive  Seedlings. 
273.  Preliminary  Report  on  Kearney  Vine- 
yard  Experimental   Drain. 

275.  The     Cultivation     of     Belladonna     in 

California. 

276.  The  Pomegranate. 

277.  Sudan   Grass. 

278.  Grain    Sorghums. 

279.  Irrigation   of  Rice  in   California. 
283.  The  Olive  Insects  of  California. 
294.   Bean   Culture  in   California. 

304.  A  Study  of  the  Effects  of  Freezes  on 

Citrus   in    California. 
310.  Plum    Pollination. 

312.  Mariout  Barley. 

313.  Pruning      Young      Deciduous      Fruit 

Trees. 
319.   Caprifigs    and    Caprification. 

324.  Storage  of   Perishable  Fruit  at  Freez- 

ing Temperatures. 

325.  Rice     Irrigation     Measurements     and 

Experiments   in   Sacramento   Valley, 

1914-1919. 
328.  Prune   Growing  in   California. 
331.  Phylloxera-Resistant    Stocks. 
335.   Cocoanut   Meal   as   a   Feed   for   Dairy 

Cows   and   Other  Livestock. 

339.  The    Relative    Cost    of    Making    Logs 

from   Small   and  Large  Timber. 

340.  Control     of     the     Pocket     Gopher     in 

California. 

343.  Cheese    Pests    and   Their    Control. 

344.  Cold   Storage   as   an   Aid  to   the   Mar- 

keting of  Plums. 

346.  Almond    Pollination. 

347.  The  Control  of  Red  Spiders  in  Decid- 

uous Orchards. 

348.  Pruning  Young  Olive  Trees. 

349.  A    Study    of    Sidedraft    and    Tractor 

Hitches. 

350.  Agriculture      in      Cut-over      Redwood 

Lands. 

352.  Further  Experiments  in  Plum  Pollina- 

tion. 

353.  Bovine   Infectious   Abortion. 

354.  Results  of  Rice  Experiments  in   1922. 

357.  A    Self-mixing    Dusting    Machine    for 

Applying      Dry      Insecticides      and 
Fungicides. 

358.  Black    Measles,    Water    Berries,     and 

Related  Vine  Troubles. 

361.  Preliminary   Yield   Tables   for    Second 

Growth   Redwood. 

362.  Dust  and  the  Tractor  Engine. 

363.  The  Pruning  of  Citrus  Trees  in  Cali- 

fornia. 

364.  Fungicidal   Dusts   for   the   Control    of 

Bunt. 

365.  Avocado  Culture  in  California. 

366.  Turkish  Tobacco  Culture,  Curing  and 

Marketing. 

367.  Methods  of  Harvesting  and  Irrigation 

in  Relation  of  Mouldy  Walnuts. 

368.  Bacterial  Decomposition  of  Olives  dur- 

ing Pickling. 

369.  Comparison     of     Woods     for     Butter 

Boxes. 


BULLETINS 
No. 


370. 
371. 


372. 


373. 
374. 


375. 

376. 

377. 
379. 
380. 

381. 

382. 

383. 

385. 
386. 

387. 
388. 

389. 
390. 

391. 

392. 
393. 
394. 

395. 
396. 

397. 

398. 


400. 
401. 

402. 
403. 
404. 
405. 
406. 
407. 


408. 
409. 


410. 


412. 


413 


414, 


Browning  of  Yellow  Newtown  Apples. 
The    Relative    Cost   of    Yarding    Small 

and   Large   Timber. 
The  Cost  of  Producing  Market  Milk  and 
Butterfat  on  246  California  Dairies. 
Pear   Pollination. 

A  Survey  of  Orchard  Practices  in  the 
Citrus    Industry  of    Southern    Cali- 
fornia. 
Results   of   Rice   Experiments   at   Cor- 

tena,    1923. 
Sun-Drying  and  Dehydration  of  Wal- 
nuts. 
The  Cold   Storage  of  Pears. 
Walnut   Culture   in   California. 
Growth    of    Eucalyptus    in    California 

Plantations. 
Growing     and     Handling     Asparagus 

Crowns. 
Pumping    for    Drainage    in    the    San 

Joaquin    Valley,    California. 
Monilia  Blossom  Blight   (Brown  Rot) 

of  Apricot. 
Pollination    of   the    Sweet    Cherry. 
Pruning     Bearing     Deciduous     Fruit 

Trees. 
Fig   Smut. 

The   Principles   and   Practice   of   Sun- 
drying  Fruit. 
Berseem  or  Egyptian   Clover. 
Harvesting    and    Packing    Grapes    in 

California. 
Machines  for  Coating  Seed  Wheat  with 

Copper    Carbonate   Dust. 
Fruit    Juice    Concentrates. 
Crop  Sequences  at  Davis. 
Cereal   Hay   Production  in   California. 

Feeding  Trials  with  Cereal  Hay. 
Bark  Diseases   of  Citrus  Trees. 
The    Mat    Bean    (Phaseolus    aconitifo- 

lius). 
Manufacture   of  Roquefort  Type  Cheese 

from   Goat's   Milk. 
Orchard  Heating  in  California. 
The    Blackberry    Mite,    the    Cause    of 
Redberry   Disease   of   the   Himalaya 
Blackberry,    and   its   Control. 
The  Utilization  of  Surplus  Plums. 
Cost    of    Work    Horses    on    California 

Farms. 
The  Codling  Moth  in  Walnuts. 
Farm-Accounting  Associations. 
The  Dehydration  of  Prunes. 
Citrus  Culture  in  Central  California. 
Stationary  Spray  Plants  in  California. 
Yield,    Stand   and   Volume   Tables   for 
White    Fir    in    the    California    Pine 
Region. 
Alternaria  Rot  of  Lemons. 
The  Digestibility  of  Certain  Fruit  By- 
products   as    Determined   for    Rumi- 
nants. 
Factors  Affecting  the  Quality  of  Fresh 

Asparagus  after  it  is  Harvested. 
Paradichlorobenzene   as    a    Soil   Fumi- 

gant. 
A  Study  of  the  Relative  Values  of  Cer- 
tain Root  Crops  and  Salmon  Oil  as 
Sources  of  Vitamin  A  for  Poultry. 
The    California    Poultry    Industry;    a 

Statistical  Study. 
Planting   and  Thinning  Distances   for 
Deciduous  Fruit  Trees. 


No. 

87.  Alfalfa. 
117.  The    Selection    and    Cost    of    a    Small 

Pumping  Plant. 
127.  House   Fumigation. 
129.  The  Control  of  Citrus  Insects. 
136.  Melilotus    indica    as    a    Green-Manure 

Crop  for  California. 
144.  Oidium    or    Powdery    Mildew    of    the 

Vine. 
157.  Control  of  the  Pear  Scab. 
160.  Lettuce  Growing  in  California. 
164.   Small  Fruit  Culture  in  California. 
166.  The  County  Farm  Bureau. 
170.  Fertilizing    California     Soils    for    the 

1918  Crop. 
173.  The    Construction    of   the   Wood-Hoop 

Silo. 

178.  The  Packing  of  Apples  in  California. 

179.  Factors   of    Importance   in    Producing 

Milk  of  Low  Bacterial  Count. 
190.  Agriculture  Clubs  in  California. 
199.   Onion    Growing   in    California. 

202.  County   Organizations  for  Rural   Fire 

Control. 

203.  Peat  as   a   Manure   Substitute. 

209.  The  Function  of  the  Farm  Bureau. 

210.  Suggestions  to  the  Settler  in  California. 
212.   Salvaging    Rain-Damaged    Prunes. 
215.  Feeding  Dairy  Cows  in  California. 
217.  Methods   for  Marketing  Vegetables   in 

California. 
220.  Unfermented  Fruit  Juices. 
228.  Vineyard  Irrigation  in  Arid  Climates. 

230.  Testing  Milk,   Cream,   and  Skim  Milk 

for  Butterfat. 

231.  The   Home   Vineyard. 

232.  Harvesting    and    Handling    California 

Cherries   for   Eastern    Shipment. 

234.  Winter  Injury  to  Young  Walnut  Trees 

during  1921-22. 

235.  Soil    Analysis     and     Soil    and    Plant 

Inter-relations. 

236.  The    Common    Hawks    and    Owls    of 

California    from    the    Standpoint    of 
the  Rancher. 

237.  Directions  for  the  Tanning  and  Dress- 

ing of  Furs. 

238.  The  Apricot  in  California. 

239.  Harvesting     and     Handling     Apricots 

and  Plums  for  Eastern  Shipment. 

240.  Harvesting    and    Handling    Pears    for 

Eastern  Shipment. 

241.  Harvesting  and  Handling  Peaches  for 

Eastern   Shipment. 

243.  Marmalade  Juice  and  Jelly  Juice  from 

Citrus  Fruits. 

244.  Central  Wire  Bracing  for  Fruit  Trees. 

245.  Vine   Pruning  Systems. 

247.  Colonization   and   Rural   Development. 

248.  Some   Common   Errors   in   Vine  Prun- 

ing and  Their  Remedies. 

249.  Replacing  Missing  Vines. 

250.  Measurement   of   Irrigation   Water  on 

the  Farm. 

252.  Supports  for  Vines. 

253.  Vineyard  Plans. 

254.  The  Use  of  Artificial  Light  to  Increase 

Winter   Egg   Production. 


CIRCULARS 
No. 


255. 

256. 
257. 
258. 
259. 
261. 
262. 
263. 
264. 

265. 
266. 

267. 

269. 
270. 
272. 

273. 

274. 

276. 
277. 

278. 

279. 

281. 


282. 

283. 
284. 
285. 
286. 
287. 
288. 
289. 
290. 
291. 

292. 
293. 
294. 
295. 


298. 

299. 
300. 
301. 
302. 
303. 

304. 
305. 
306. 

307. 
308. 


Leguminous  Plants  as  Organic  Fertil- 
izer  in    California   Agriculture. 

The   Control   of  Wild   Morning   Glory. 

The  Small-Seeded  Horse  Bean. 

Thinning  Deciduous   Fruits. 

Pear  By-products. 

Sewing  Grain  Sacks. 

Cabbage  Growing  in  California. 

Tomato  Production  in  California. 

Preliminary  Essentials  to  Bovine 
Tuberculosis  Control. 

Plant  Disease   and  Pest  Control. 

Analyzing  the  Citrus  Orchard  by 
Means  of   Simple   Tree   Records. 

The  Tendency  of  Tractors  to  Rise  in 
Front;    Causes   and  Remedies. 

An  Orchard  Brush  Burner. 

A  Farm  Septic  Tank. 

California  Farm  Tenancy  and  Methods 
of  Leasing. 

Saving  the  Gophered  Citrus  Tree. 

Fusarium  Wilt  of  Tomato  and  its  Con- 
trol by  Means  of  Resistant  Varieties. 

Home  Canning. 

Head,  Cane,  and  Cordon  Pruning  of 
Vines. 

Olive  Pickling  in  Mediterranean  Coun- 
tries. 

The  Preparation  and  Refining  of  Olive 
Oil   in    Southern   Europe. 

The  Results  of  a  Survey  to  Determine 
the  Cost  of  Producing  Beef  in  Cali- 
fornia. 

Prevention  of  Insect  Attack  on  Stored 
Grain. 

Fertilizing  Citrus  Trees  in  California. 

The  Almond   in   California. 

Sweet  Potato  Production  in  California. 

Milk  Houses  for  California  Dairies. 

Potato   Production   in   California. 

Phylloxera  Resistant  Vineyards. 

Oak  Fungus  in  Orchard  Trees. 

The  Tangier  Pea. 

Blackhead  and  Other  Causes  of  Loss 
of  Turkeys  in  California. 

Alkali  Soils. 

The   Basis   of   Grape    Standardization. 

Propagation   of   Deciduous   Fruits. 

The  Growing  and  Handling  of  Head 
Lettuce  in   California. 

Control  of  the  California  Ground 
Squirrel. 

The  Possibilities  and  Limitations  of 
Cooperative  Marketing. 

Poultry  Breeding  Records. 

Coccidiosis  of  Chickens. 

Buckeye  Poisoning  of  the  Honey  Bee. 

The   Sugar  Beet  in   California. 

A  Promising  Remedy  for  Black  Measles 
of  the  Vine. 

Drainage  on  the  Farm. 

Liming  the  Soil. 

A  General  Purpose  Soil  Auger  and  its 
Use  on  the  Farm. 

American   Foulbrood   and  its   Control. 

Cantaloupe  Production  in  California. 


The  publications  listed  above  may  be  had  by  addressing 

College  of  Agriculture, 

University  of  California, 

Berkeley,  California. 

13m-5,'27 


